Schlieren Imagery
Schlieren imagery is an imaging process whereby changes in the angular deviation of an optical beam are transformed into changes in the intensity of the Schlieren image. A Schlieren imaging apparatus contains two image planes, a source image plane, where an image of the optical source is produced, and an object image plane, where an image of the object illuminated by the optical source is produced. Refractive index gradients in the object produce angular deviations in the optical beam. These angular deviations are transformed by the Schlieren optics into deviations in the position of the optical source image. A filter, e.g. a knife-edge, placed in the plane of the source image causes the angular deviations to produce intensity shifts in the object image.
There are many variations in the basic Schlieren concept. For example, dark field microscopy is a form of Schlieren imagery. However, all Schlieren apparatus utilize a finite size source with some preset geometry and a mask (typically at the source image plane) which causes amplitude variations to appear in the object image as a result of refractive index gradients in the object. For the following discussion, a knife-edge type Schlieren system as described in Schlieren Methods, L. A. Vasil'ev, pages 86-149 is assumed. For small angular deviations, it can be shown that the change in object image intensity is linearly related to the refractive index gradient in the illuminated object by ##EQU1##
where k is a constant relating to the source size, object transmission and reflection parameters, and focal powers of the imaging system, I.sub.0 is the illumination intensity, n is the refractive index of the object, z is in the propagation direction of the optical beam, and x is the direction perpendicular to the knife-edge and z.